I thank everyone that replied, but most if not all the answers do not take into account the limitations of NOT changing the current (192.168.1.1) address of the gateway and/or are anyway using the "added router in the middle" option where the proposed Microtik hex router is replaced by some other brand/make/model.

I understand (a little) the way multi-WAN routing works, but due to the exposed limitations that cannot work.

The whole problem revolves around the fact that the (hypothetical) device (if it is a sort of router) has an own address of 192.168.1.1 and that it should be able to route/switch to one of the three DSL/4G router/modems each one having the same 192.168.1.1 address.

I am now thinking of a ethernet relay like the ones used in home automation, similar to this one (example):

https://relaypros.com/Relay/Relay/ETHERNET_RELAY_CONTROLLERS

but I read somewhere that it is not a good idea to use the relay contacts on the actual ethernet cable, so I could use one of them to control the power of the three routers/modems, but this would introduce a (bearable, still ...) delay as the routers take some time (tens of seconds, one is particularly slow and takes over a minute) to boot and establish connection.